In microorganisms, L-arginine is biosynthesized from L-glutamic acid through eight reaction steps. L-ornithine and L-citrulline are intermediates on the L-arginine biosynthetic pathway. Biosynthesis of L-arginine, L-ornithine and L-citrulline is regulated similarly to that of other amino acids.
In coryneform bacteria, for example, transcription of an operon composed of genes encoding enzymes responsible for L-arginine biosynthesis (hereinafter abbreviated as arginine operon) is repressed by the arginine repressor (hereinafter referred to as ArgR) (see patent publication No. 1). It is known that in coryneform bacteria N-acetylglutamate kinase, which is the second enzyme on the biosynthetic pathway from L-glutamic acid to L-arginine (EC: 2.7.2.8, hereinafter sometimes abbreviated as ArgB), is subject to feedback inhibition by L-arginine (see non-patent publication No. 2).
L-Arginine, L-ornithine and L-citrulline are produced using microorganisms as well as other amino acids, and studies have been conducted, as in the case of other amino acids, to enhance the productivity of these amino acids by the use of means such as mutation or recombinant DNA techniques.
For example, there is a report that Corynebacterium glutamicum K65 (FERM BP-1115) having enhanced arginine productivity was obtained by transforming Corynebacterium glutamicum with a DNA fragment containing a gene responsible for arginine biosynthesis and then carrying out mutagenesis (see patent publication No. 2).
Corynebacterium glutamicum wherein the repression of arginine biosynthetic enzymes is cancelled (see non-patent publication No. 1) and Corynebacterium glutamicum wherein the repression of arginine biosynthetic enzymes is cancelled, feedback inhibition by L-arginine is desensitized and membrane permeability of L-arginine is enhanced (see non-patent publication No. 3) have also been obtained by mutagenesis.
Coryneform bacteria in which DNA encoding ArgR is destroyed (see patent publication No. 1) have been obtained by recombinant DNA techniques.
However, there has been no report so far about what mutation should be introduced into DNA encoding ArgB or ArgR to obtain a strain in which mutations are introduced into both of the DNAs respectively encoding ArgB and ArgR and which has enhanced L-arginine productivity.    Patent publication No. 1:            Japanese Published Unexamined Patent Application No. 51790/02            Patent publication No. 2:            Japanese Published Unexamined Patent Application No. 79597/88            Non-patent publication No. 1:            Agricultural & Biological Chemistry, vol. 43, p. 105-111 (1979)            Non-patent publication No. 2:            Journal of Bacteriology, vol. 91, p. 617 (1966)            Non-patent publication No. 3:            Agricultural & Biological Chemistry, vol. 36, p. 1675-1684 (1972)        